On Jun 15, 2:10 pm, "Nick" wrote:
I was under the impression that a 1/4 wave was approx 0 dBd, a
5/8ths was approx 3dBd,
and a collinear dual 5/8ths was approaching 6dBd
Nick


Most the textbooks rate the 1/4 wave GP at .3 dbi.
The 1/2 wave at 2.1 dbi or 0 dbd.
The 5/8 GP appx 3.3 dbi, or 1.2 dbd.
The dual 5/8 collinear appx 5.1 dbi or 3 dbd.
But.... If you model the antennas, the sloping radial
1/4 GP actually does a bit better.. Appx 1.8 dbi,
which is within .3 db of the 2.1 dbi you would see
from a 1/2 wave.
BTW, these are "free space" numbers.
I don't like to confuse people by adding ground
effects... Besides, if you do model over ground,
you will see the same comparison spread..
It doesn't really change anything..
The gain from a 5/8 GP can be all over the map depending
on the radial length used.. Most 5/8's use 1/4 wave radials.
No bueno.
At an old page I put up, you can see the effect of radial
length on the performance of the 5/8 GP.
http://web.wt.net/~nm5k/acompari.htm
Many people rag on the 5/8 antenna, but the largest
problem is due to an old lousy obsolete design
which uses the short 1/4 wave radials.
If you use longer radials, such as 5/8 or 3/4 you
will see the rated gain you would expect from a 5/8 GP.
If you use 1/4 wave radials, you can often see less gain
at the desired angles than either the 1/2 wave, or 1/4 GP.
Of course, all these examples assume no common mode
currents on the feedline.. On VHF/UHF, decoupling of the
feedline can be as important as the antenna design itself.
So if I were to build say a dual 5/8 collinear, or a 5/8 GP
with steep 5/8 radials, I would add a decoupling section
below the lower element.
IE: like an isopole, or whatever.. It used a lower 1/4 wave
cone as the decoupling device. You could also add a
section of 1/4 wave radials for decoupling..
6dbd from a dual element vertical?
Good luck..A 3 el NBS yagi is only good for about 7.3 dbd,
and it will thrash a dual 5/8 collinear.
If the height is limited, I would use the highest gain antenna
I could scrape up. Even if you have obstructions, it's going
to do a heck of a lot better than most any vertical at the
same height.
MK